It is known that under certain conditions, solid solutions of two or more quinacridone pigments having excellent properties can be obtained. Such solid solutions are described for instance in U.S. Pat. Nos. 3,160,510; 3,298,847; 3,647,494 and 3,681,100.
Many processes describe the preparation of quinacridones starting from 6,13-dihydro-quinacridones by oxidation with various oxidants like, for example, nitrobenzene derivatives, such as m-nitrobenzene sulfonic acid sodium salt; selenium; sulfur; iodine; oxygen or air. In another method, quinacridone is obtained by cyclization of 2,5-diphenylamino terephthalic acid in polyphosphoric acid. These processes are not environmentally friendly because they generally require large quantities of organic solvents, strong bases or strong acids. Furthermore, they often employ environmentally unfriendly oxidants which afford large quantities of waste materials, such as the reduced organic by-products.
Copending Application Ser. No. 601017,050 describes an oxidation process for the preparation of quinacridone pigments, including quinacridone solid solutions by (A) premilling the corresponding 6,13-dihydroquinacridone followed by (B) oxidation in an aqueous basic medium with hydrogen peroxide as the oxidant. Although it provides environmental advantages versus the state of the art, this process has the drawback of being a two step process. Thus, it is more difficult to control to obtain a consistent product and it requires expensive milling and synthesis equipment for manufacturing.
The present invention describes a process for the preparation of solid solutions comprising quinacridone, which is also referred to as 5,12-dihydroquino[2,3-b]acridine-7,14dione, a well-known pigment having the formula (I) ##STR1## and 6,13-dihydroquinacridone of the formula (II) ##STR2## by oxidation of a 6,13-dihydroquinacridone in an aqueous basic alcoholic medium using hydrogen peroxide as the oxidant and the use of the solid solutions as red organic high performance pigments.
Quinacridone is known to exist in three major polymorphic modifications: an alpha form which is disclosed in U.S. Pat. No. 2,844,484, a gamma form which is disclosed in U.S. Pat. No. 2,844,581 and U.S. Pat. No. 2,969,366, and a beta form, which is described in U.S. Pat. Nos. 2,844,485 and 4,857,646. The alpha and gamma polymorphs are red pigments, while the beta polymorph has a violet to magenta hue.
The gamma polymorph itself is known to exist in three forms, a bluish-red gamma-II form which is disclosed in U.S. Pat. No. 2,844,581, and two yellowish-red forms which are designated as the gamma-I and gamma-III forms and disclosed in U.S. Pat. Nos. 3,074,950 and 5,233,624, respectively.
Each of the three gamma forms of quinacridone possesses a distinct x-ray diffraction pattern. The bluish-red gamma-II form shows an x-ray diffraction pattern with three strong lines at 6.6, 13.9 and 26.3; five medium lines at 13.2, 13.4, 23.6, 25.2 and 28.3; and two weak lines at 17.1 and 20.4.degree. 2.theta. double glancing angles. The yellowish-red gamma-I form shows an x-ray diffraction pattern with three strong lines at 6.6, 13.9 and 26.5; three lines of medium intensity at 13.2, 13.5 and 23.8, and four weak lines at 17.1, 20.5, 25.2 and 28.6.degree. 2.theta. double glancing angles. The yellowish-red gamma-III form shows an x-ray diffraction pattern with four strong lines at 6.7, 13.3, 14.0 and 26.6; one medium strength line at 13.6 and seven relatively weak lines at 17.2, 20.6, 21.9, 24.0, 25.3, 28.1 and 28.80 28 double glancing angles.
Several processes are described for the preparation of gamma quinacridone pigments, for example, starting from quinacridone crude by milling, followed by recrystallization in certain organic solvents or by using precipitation processes such as, for example, precipitation from basic DMSO or polyphosphoric acid.
The present invention is based on the discovery that a high chroma quinacridone/-6,13-dihydroquinacridone solid solution having the crystal structure of a gamma-I quinacridone is obtained by oxidation of 6,13-dihydroquinacridone in an aqueous basic alcoholic medium in the presence of a quinone catalyst and using hydrogen peroxide as the oxidant if a special form of a 6,13-dihydroquinacridone salt is generated prior to the addition of the oxidant. It was additionally discovered that a highly saturated and opaque quinacridone/6,13-dihydroquin-acridone solid solution having the crystal structure of a gamma-II quinacridone can be prepared by essentially the same process if a catalytic amount of an aromatic nitro compound is added prior to the addition of the hydrogen peroxide.
Thus, the inventive process is valuable because two different quinacridone high performance pigments having different hues can be made by practically the same environmentally friendly process route.